Design, Installation, and Testing of Hollow Bar Tiebacks Used as Excavation Support-Paper

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The paper describes the design, installation, and testing of self-drilling hollow bar tiebacks used to support two deep excavations for culvert replacement in East Ryegate, Vermont. The excavations—60 feet deep through a highway embankment and 70 feet deep through a railroad embankment—had to remain narrow so traffic could continue on temporary bridges. Their support system combined sheet piles, hollow bar tiebacks in the upper, looser soils, and hollow bar soil nails in the denser lower soils.<br /><br />Hollow bar anchors were selected because cement grout could serve as drilling fluid, and the anchor could be installed without removing drill steel—reducing the risk of ground loss in granular soils. A key challenge was creating a dependable unbonded free length. The team used schedule 40 PVC sleeves over the bars, coated the sleeves internally with debonding oil, and carefully sealed their ends to the couplers. Grout was also flushed from the first three feet behind the wall to reduce the “donut hole effect,” in which a continuous grout column can distort test results by transferring load outside the intended bond zone.<br /><br />All tiebacks received proof tests; selected anchors received performance tests. Testing generally followed Post-Tensioning Institute recommendations, with adjustments to account for the behavior of hollow bar systems. Early tests revealed stick-slip movement, incomplete free-length debonding, and apparent negative creep in shallow anchors. Improved sleeve preparation and installation practices reduced these issues. In very loose soils, larger initial movements were sometimes needed to mobilize the bond zone, while anchors in denser soils behaved more conventionally.<br /><br />Monitoring showed maximum wall movements of approximately 2–3 inches, or about 0.3% of wall height. Surface settlements were roughly 1–2 inches, and only minor roadway and railroad adjustments were needed. The project demonstrates that hollow bar tiebacks can efficiently support deep temporary excavations, provided free-length construction, grouting, and testing are carefully controlled.

Keywords

hollow bar tiebacks

self-drilling anchors

deep excavations

culvert replacement

sheet piles

free length debonding

proof testing

performance testing

grout column effect

temporary excavation support